Glow discharge apparatus



April 2, 1957 B. BERGHAUS ETAL 2,787,730

GLOW DISCHARGE APPARATUS Filed Jan. 18, 1951 I N VENTOR BERNHARD BERGHAUS HANS eucsx ATTORNEY United States Patent GLOW DISCHARGE APPARATUS Bernhard Berghaus, Lachen, and Hans Bucek, Zurich, Switzerland; said Bucek assignor to said Berghaus Application January 18, 1951, Serial No. 206,675

6 Claims. (Cl. 315-111) It is well known that in certain forms of gaseous discharges, particularly glow discharges, such as are used for carrying out technical processes, oscillations of plasma occur. By technical processes are meant measures based on physical effects such as diffusion e. g. chromium in steel surfaces, decarbonising etc., based on chemical effects such as reactions (synthesis) and on physicochemical effects (e. g. nitriding of steel, carburising, etc.). In such processes the objects may be subjected to the action of plasma. The name plasma designates, as is known, a mixture of partially and completely ionised gas molecules, electrons and secondary electrons which fills the space between the cathode glow and the glowing film on the anode. Plasma oscillations moreover occur in low-pressure are as well as in glow discharges. Besides the form of discharge, other factors affecting the plasma oscillations are the temperature arising as well as the spacing and configuration of the electrodes, whilst their frequency is dependent in the first place on the density of the charge carriers in the electric gaseous discharge. In certain closely investigated cases it was found that the frequencies arising are approximately proportional to the square root of the ion or electron density. The factors which lead to the formation and influencing of oscillations of this kind are extremely numerous and are still not fully apparent, so that in the present state of scientific and technical knowledge, there are still no means and possibilities known whereby one can control the plasma oscillations immediately after they form and control their frequency as they originate, once their formation has been accepted. Besides this, it must be taken into account that otherwise effective means, such as choice of electrode spacing, the gas pressure and the temperature etc. and the possibility of influencing their mutual, functional relations are ruled out from the start in carrying out technical processes, since the desired final product or the work-piece under treatment is the deciding factor in the process.

Furthermore a characteristic feature of these very technical productions and working processes is that even at very low pressures such as were employed at first in practical operations, that is pressures in the order of magnitude of fractions of a millimeter up to not more than 3 mm. Hg, there occur high specific loads at the.

surface of the object with a correspondingly high energy conversion. However, it has recently been proposed to employ considerably higher pressures havingan order of magnitude of 3 to 100 mm. Hg and even beyond. Thus it became desirable to subject the energy conditions in the gaseous discharge space to more minute investigation with a view to ascertaining Whether the delivered electrical energy is convertedas is highly desirableas completely as possible into gaseous discharge energy, thereby becoming available for the process being carried out. In doing this, the surprising discovery was made that, for example in the treatment of machine parts of certain shape, failures arise to a far greater extent than one would have assumed from previous considerations or computations e. g. allowing for differences in the composition of the materials. The cause of these failures was found to reside in high frequency effects caused by plasma oscillations. An ac cidental observation was also found to be of assistance when, in changing to more powerful energy conversions by employing a pulse system, resonator elements inserted by chance in the gaseous discharge space exhibited a behaviour which can only be attributed to powerful oscillations of very high frequency caused by the high charge carrier concentration created. As a matter of fact, after ascertaining these revealing facts it is clear that effects such as resonances or cavity resonances etc. on the workpieces (and also in unfavourable cases, the high frequency effects which may give rise to an appreciable withdrawal of energy from the total electrical action initiated in the gaseous discharge space) must have a deleterious action on the conduct of the process concerned. This is particularly the case when further portions of the discharge space or conductor networks associated with it are allowed to be excited into intensive high-frequency oscillations outside the actual region of origin of the oscillations through resonance effects. Then again, the oscillations assist the technical processes as such, only in relatively rare and not readily controllable cases. It will always depend on the position and shape of the work-pieces and a large unpredicable number of other factors and effects, as well as their functional consequences, as to whether charge carriers of higher energy arising for example as the result of plasma oscillations, will act to the advantage of the entire process during the ionisation of a treating gas at the surface of the work-piece, orwhat is much more likelywill appreciably diminish the economy and total efficiency of the process. Here it is not only a question of the charge carriers acquiring an energy differing from a normal mean value and thus being likely to bring about uncontrollable effects; there is still a further factor of uncertainty produced in the technical handling of objects by the fact that the distribution of the charge carriers over the object is no longer homogeneous but is locally concentrated at certain places whilst being considerably diminished at others. For example in the nitriding of a work-piece it may happen that normally nitrided, i. a. extremely hard surface portions alternate with less hard or unchanged parts of the, material. When the work-pieces are liable to be excited into resonance by oscillations, for example due to their considerable length or other appropriate shaping, there is the further circumstance that even at places where the charge carriers are not directly influenced by plasma oscillations, irregularities in the treatment of the surface of the object may occur if it is possible for standing waves with voltage maxima and minima to develop. Only in certain and generally unapplicable cases willit be possible to make the higher energy contained in the developing charge carriers available with intensified effect at the work-piece.

In speaking of the total or overall efficiency of the process in this connection, it is intended to allow for both the electrical performance factors in the usual sense of the term and also the quality of the final product.

Finally it should be noted that uncontrolled highfrequency waves propagate themselves also to the space outside the devices, apparatus and installations used for carrying out the technical processes and due to their action, which may last throughout the entire working period, they may produce unforeseeable and uncontrollable disturbing influences on their non-living and (in the case of ultra-short waves for example) living environment.

Proceeding from these observations and discovered facts, the methods proposed according to the invention for carrying out technical processes by means of gaseous, preferably glow discharges, are characterised by the fact that the energy imparted to the charge carriers in the gaseous atmosphere is made available to an increasing extent for conducting the technical processes themselves by diminishing the effects of high-frequency oscillations forming in the discharge space. In the main, this availability is made possible by conducting the processin such a way that high-frequency oscillations arising in the discharge space are practically completely eliminated. However in most cases itwill not be necessary to go so far. For all practical purposes, it will be sufficient if the purely spatial extension of high-frequency oscillations forming in the discharge space, towards the surrounding area outside same is diminished to such a degree as to ensure the utilisation of the energy imparted to the charge carriers in the gaseous atmosphere. Only in special cases will this extension be totally eliminated. The surrounding area extending to the limits of the discharge space may be distinguished from the surrounding area outside such limits and the method identified according to the invention may be applied in each of the two cases.

In the high-frequency art, a number of measures will be known, whereby the method characterised of the invention can be carried out in its details. At least one of the following means may be employed to ensure the carrying out of technical processes in gaseous discharges: choosing the natural electrical frequencies of all oscillatory networks or structures in the system in non-coincidence with the frequencies given by the plasma oscillations, increasing the attenuation of all these networks or structures as far as possible, further, the completest possible screening of the immediate vicinity of the place of origin of the oscillations in the manner of a Faraday cage, this including shunting the parts in question by a condenser or resistance, short-circuit connections between same, and finally blocking the high-frequency oscillations emerging from the discharge vessel against further propagation to the environment, by using filtering means in the electrical connections whilst at the same time provid' ing tapping points of known potential.

In particular the method will first be mentioned for eliminating or substantially limiting resonant ranges between high-frequency oscil ations and oscillations which can be originated or intensified by excitation, thatmeans, to detune, since there are oscillatory networks or structures outside the region of origin of the plasma oscillation in every case. Amongst such structures capable of oscil lation may be mentioned chiefly the boundaries of the discharge space, say in the form of the walls of the gaseous discharge space, and its base plate, which for practical reasons necessarily consists of several parts. Mention must also be made of all electrically effective parts provided in such discharge vessels, such as the lead-in devices i. e. the electrodes, also the work-piece itself,,its screening elements, its cooling and heating jackets supports and suspensions for the work-piece, etc.

Further possibilities are the provision of means forming a high-frequency capacitative short-circuit or a connection through ohmic resistances or both. Thus it. is proposed that voltage-carrying or only mutually insulated parts will be short-circuited by capacitors or electrical resistances will be interposed between these parts. In the first place this will be appropriately carried out with respect to the current lead-in and lead-out devices, since these parts passing through the walls of the gaseous dis charge vessel, and brielly designated as current leads, may conduct the higlnfrequency outwards and moreover may act as resonators. What is stated with regard to the leads applies appropriately to all voltage carrying circuit elements, i. e. to all parts of the gaseous discharge vessel which are connected to an electrode or act as such. Moreover. the work-piece is connected to one of the electrodes, usually to the cathode and in this case is covered with negative glow. Here the loads per unit area may under certain circumstances assume exceedingly high amounts depending on the prevailing working conditions, so that the charge-carrier concentrations attain values leading to the creation of high-frequency with all short wave lengths. Thus for example, the formation of very powerful oscillations in the, centimeter wave region has been confirmed. These observations and factual findings have of course led to the fundamental principles governing the design of the means, and their particular appropriate arrangement for carrying out the methods envisaged, which will be gone into in detail later. In every case it will be necessary to take into account that the frequency of. the oscillations is variable due to changes in loading and all further influences on the change in the ion and electron density.

The electrical resistances to be inserted in circuit may be of various kinds, in particular of course conventional wire resistors or those of the composition or layer type. in addition to these, however, directly available structural parts of the system may be used as resistances, for example connecting channels of the heat-exchange means for the walls of the gaseous discharge vessel. These heat-exchange means are generally in the form of cooling media but they may also act as heating media and they may also be present or available for cooling the electrodes. In a further embodiment of the invention, therefore, it is proposed to connect voltage-carrying conductors or only mutually insulated parts, or both such conductors and insulated parts electrically via streams of a heat-exchange medium in contact with them, in so far as this medium possesses electrical conductivity of sufiicient magnitude. Throughout, the cathodes and sometimes also the anodes, are traversed by cooling medium on account of their heavy thermic loading, so. that it is found particularly advantageous to connect the conductors, and possibly the insulated parts, electrically over the stream of a heatcxchange medium which contacts them successively.

Mention must be made of further means for limiting the generated high-frequencies in their action on the discharge vessel namely the connecting of parts at equal potential via metallic conductors or the connecting of conductors of different potential via resistances. In cases Where capacities are used already in by-passing arrangements, it may be necessary in certain circumstances to provide additional attenuating resistances. More diiilcult is the arrangement and proportioning of resistances between parts of high potential differences. In this case, the already described use of the cooling medium channels as electrical connections has been found particularly successful, since heat generated in these resistances is produced in the cooling medium itself and is led away.

By the adoption of measures of the kind described, the interior of the gaseous discharge vessel, i. e. the gaseous discharge space, is sealed off to the, greatest possible extent after the manner of a Faraday cage. In order to obtain definite potential conditions with respect to the surroundings, a satisfactory grounding of suitable points may be established. As a special case of the above grounding, mention may be made of connecting the current source and the bounding elements of the gaseous discharge space to a common electrical tapping point. The tapping point may however be linking the positive or negative pole of the current transmission system with the mass of the apparatus. Likewise, it is practicable to employ a symmetrical current source and use the centre of the voltage supply as the tapping point. In this case. the proposed measures apply to all potential-carrying points of the current source itself, that is, the ends of same must be short-circuited to the tapping point by means of condensers for high frequency. Furthermore known filtering means may be employed to. keep the highfrequency away from the current leads or from the entire current supply system. For this purpose it is sutficicnt to interpose ohmic resistances or inductive impedances, in

the leads, to the discharge space. Thesemust of course be. proportioned to suit. the currents generated. They have the advantage that on the one hand the high-frequencies. arising in the current source itself (as generated for example by mercury vapor rectifiers.) do not pass into the discharge space, whilst on the other hand the resistances. and chokes. placedin circuit are useful not only as. effective high-frequency impedances but as smoothing impedances for the gaseous discharge. The factors for designing these. circuitelements need no. further discussion, since they: areknown already as standard measures familiar in the electrical and high-frequency art.

The devicesv for carrying out the. method are characterised by the arrangement ofv means for diminishing highfrequency oscillations generated. in the discharge space.

These. means. may be. realised bygiving the discharge vessel a form suppressing cavity resonances, the asymmetricalshaping of the vessel. and the asymmetrical arrangement ofelectrically elfective. arts. at the interior of. same. being found particularly suitable. It. is also of advantage to dispense with structurally simple formations or; to avoid geometrically simple configurations of itszwall surfaces. This shaping of the gaseous discharge vesselis, most effective. when it: avoids. wall portions which are'paralleL and which face each other at. the distance of half av wave-length or, a multiple thereof, and. it. largely damps. the. tendency to. excitation into resonance in. an E-mode or H-mode wave as a cavity oscillator. Thus, stiffening ribs. which are provided to resist the external atmosphere pressure may contribute to the attainment oftheobjectin questiomby an. irregular shaping. and arrangement, achieved for example by alternate spacing. Conical shaping in.round vessels ornon-parallel wall formations in. square ones are. further measures, and again it isyofx advantage to shape them asymmetrically. The same also applies to. the arrangement of electrically active parts in general. Asymmetrical electrode arrangements and forms are also advised. The conditions in the inner Space. of the discharge vessel: also change from case to case by the arrangement and mounting and dimensions of the workpiece and the mutual and relative position ofthe latterwith respect to. the walls of the discharge VCS? sel,. Thus, the asymmetrical disposition of the Work.- piece itselfmust be considered as the simplest case. Shortcircuiting by-capacities can.be realised inthe usual manner well. known in.cir.cuitry, through by-passing electrical leads andgportions of the discharge vessel or the current supply,- system with. condensers. Owing; to the very high frequencies possible, non-inductive types of capacitors should be used. Attention should also be paid to their voltage; breakdown strength, because in. addition to the high-frequency, voltages of the order of. several kv. are; appliedzat the electrodes. In order to make the HF oscillations-ineffective, in the closest vicinity of their area of origin, it hasbeen. found a particularly effective and constructionally simple arrangement to make the lead.- in deviceitself asacapacity betweenwall and conductor. With; this; object the (usually cylindrical) outer face. of the current-carrying conductor is surrounded at the smallest possible distance by corresponding opposing faces of the other electrodetthe latter being generally the Wall of the; gaseous.- discharge vessel. To increase the capacity, the lengths of. these. faces opposing each other at a small distance are made; as great as possible in the direction. of therconductor. Having regardto the order of magnitude Ofi'the: oscillations which infact reach into the ultra-highfrequency region; the spacing and other dimensions of the capacity forming. faces and of the-conductors, etc. are so:.chosen as to prevent with certainty the creation of oscillatonyi states-at these parts and in interstices which could lead toa. transfer of highrfrequency-energy outwards by coupling: effects; I

Theidischarge vesselszconsist throughout of two or more parts in orderi togive access to the interior for the purpose of facilitating the changing of the charge or workpiece undergoing treatment. Also: doors or covers are provided for sluice valves, manholesetc. The most satisfactory solution for mutuaily sealing. these parts is by using packings. These are profile packings, usually of rubber, laid ingrooves and pressed, for example, against a flat surface. These. parts must be. electrically connected so that on the one hand they carry a definite but generally equal potential and on the other hand prevent the formationof high-frequency standing waves at such electrically insulated parts.

Inorder to produce satisfactory conditions with regard to the high-frequency, it is necessary to by-pass with electric conductors preferably at several suitably distributed points. Also, parts of the gaseous discharge vessel as Well as the electrodes are water-cooled, i. e. made doublewalled. The water system is so formed that its port-ions (preferably in the sequence from the hottest to the coldest portion) are successively delivered into the same stream of cooling medium. The portion developing the greatest quantity of heat to' be dissipated, is generally the cathode support. Then the various parts of the gaseous discharge vessel are. traversed. The water columns in the various conduits are appropriately used for bridging the potential= differences. Consequently the water conduits cannot be made of metal but must be composed of insulating materials. Rubber hose connections aretherefore used. It is of advantage if the coolant supplies for anode and cathode are not separated, say by using different electrically insulated pumps and drives for same, but if the liquid channels are used as connecting resistances. It has beenv found of advantage for the cooling water hoses between parts at different voltage not to take lengthy circuitous routes, since the. cooling fluid losses from the supplying current source aresufficiently small with respect to the total: power converted, which is of the order of several kw.

The drawing illustrates a satisfactory embodiment of the invention by way of example, and shows. the construction of the discharge vessel, the electric circuit and the cooling water system.

The dischargevessel consists of a detachable, doublewalled top portion 1- and a double-walled base plate 2; A packing. 3 extending over the entire periphery of the contact surface is provided between the two parts 1 and 2/. An evacuating pump 4 driven by the electric motor 5 draws the gas out ofv the space enclosed by the discharge vessel. through the pipe 6. A shut-off valve 7 serves to seal off the discharge vessel 1 fromthe evacuating pump 4' completely; The space enclosed by the walls 1 is supplied with any suitable treating gas from a gas cylinder 8 through a pipe 9. provided with a. shut-off device 10. The cylinder 8, is provided in well-known manner with. a pressure reducing valve, 11 and pressuremeasuring devices 12 before and behind this valve.

The drawing first of all illustrates a construction of the discharge vessel. 1 which damps out the formation of stable oscillatory states. For this purpose it is designed as a hat-shaped hollow body with a sioping top at 13 and conical sides at 14, so that it cannot act as a cavity resonator, or at least only in extreme cases. This is also conditioned by arranging strengthening ribs 15 to 18 which are provided with different spacing and different widths. Not only the gaseous discharge vessel is given this asymmetrical construction in this Way; the electrically active parts of the discharge vessel are also shaped or arranged asymmetrically. To meet the requirement for the current leads to take the form. of anode and cathode, the lower electrode is disposed as a work-piece support and cathode. Opposite the latter is the anode Which is given an unsymmetrical position in this case by an arrangement oblique to the longitudinal direction of, the cathode. The anode consists of a central conductor 19 which at 20 branches. into the electrode arms 21, 22 again of unsymmetrical construction and arrangement. The conductor 19 is held in 7 an insulator 23 which is secured in the vessel wall 13. The further parts of the anode will be discussed in another connection later.

The cathode, in contradistinction to the anode, is designed to be cooled. It consists substantially of a hollow cylinder 24 which at the same time serves as a support for the work-piece 25. The hollow cathode portion is mounted in an insulator 26 and through the latter is secured to the base portion 2 of the discharge vessel. The hollow cylinder passes into a connecting flange 27 for the supply conduit 28 for the cooling medium. A centrally disposed pipe 29 serves for leading off the cooling medium.

Both the anode lead-in device 19, 23 as well as the cathode lead-in device 24 are constructed as condensers according to the spirit of the present invention. With this object, the central portions of the two electrodes are surrounded by tubular portions 34}, 31 enclosing them with a narrow spacing in-between. The capacitors 19, 39 and 24, 31 thus formed constitute short-circuiting capacitators between the conductors 19, 24 and the walls 13, 2 of the gaseous discharge vessel.

Current is supplied to the two electrodes from current sources 32, 33 which are connected to the above mentioned electrodes 19, 24 through the conductors 34, 35. These supply leads 34, 35 contain ohmic resistances 36, 37 and inductive impedances 38, 39 to prevent the propagation of any high frequency oscillations generated or to produce an appropriate attenuation in portions of the conductors acting as oscillatory circuits. In addition, it is necessary to prevent high frequency generated in the current source from having access to the discharge vessel and vice versa. Altennatively chokes may be used alone or resistances alone or these elements may be used in one of the leads only. These effective highfrequcncy impedances may be used as smoothing means for the gaseous discharge. In the present example the midpoint of the current sources 32, 33 is connected to the envelope of the gaseous discharge vessel. This neutral tapping point 40 is also grounded at 41. To obtain definite potential conditions, it is possible, instead of the mid-point symmetrical tapping of the current source 32, 33 at 40 to select one of the two ends of the voltage sources 32, 33 as the tapping point for the electrical system. This is illustrated by way of example for the tapping point 35, where a connection to ground is effected at 42. Similarly the other end of the current source can be chosen as the tapping point, as also the parts 19, 24. In all cases, to obtain established potential conditions with respect to the surroundings it is of advantage to ground the tapping point.

Also the poles of the current supply system 34, 35 are short circuited for high frequency by the condensers 43, 44, 45. Possibly, these could be connected in the same way to the earthing point 42.

Additionally to make the electrode leads as lay-passing members acting as capacitative leaks for high-frequency oscillations further capacities are provided as external circuit elements, as for example the condensers situated at 46, 57, and 48.

The drawing finally illustrates how it is possible to attain the by-passing of voltage-carrying parts with ohmic resistances which are formed by the stream of a heat exchange medium. As already mentioned the cathode 24 is cooled and the conduit 29 is provided to lead off the cooling medium. The coolant circulation now passes over the tubing 49 to the connection 50 of the cooling space 51 formed by the double-walled discharge vessel. The coolant leaves the cooling space 51 through the connection 52, the tubing 53 joined to it, and passes in this way into the cooling space 54 of the base plate 2 of the gaseous discharge chamber. After the cooling medium has absorbed the heat of cooling it is led off via the connection 55 and the pipe line 56 attached to it. In this way, different parts and also at different potential are connected in the circulatory system by streams of cooling medium; for example the tubing 49 connects the cathode 24 to the upper portion of the discharge vessel, which has a different potential. For this reason the tubing 49 takes the form of an electrically insulating hose connection, for example rubber hose, so as to prevent direct short circuiting of the parts at diiferent potential. Consequently, besides the condensers 47, 48 there is also an ohmic connection between the cathode and the walls of the discharge vessel via the column of liquid in the hose 49. Thus there is not only the advantage that any heat generated in the circuit is led off immediately but also that there is a considerable simplification in the coolant circulatory system. As an electrical circuit combination it also constitutes in this way a highly attenuated network.

The already mentioned rubber packings 3 insulate the upper portion of the gaseous discharge vessel from the base plate 2. In order to prevent or diminish the emergence of high-frequency oscillations, these two portions are bridged over direct by the metallic conductors 57, 58.

Hence it is clear that the measures indicated substantially diminish the effects of high-frequency oscillations forming in the discharge space. By a partial or complete combination of the measures indicated, it is possible practically to eliminate the effects of the highfrequency oscillations arising in the gaseous discharge space.

However it is at least possible to ensure that the extension of these high-frequency oscillations forming in the discharge space, towards the surrounding area outside the region of origin of same, is diminished so that the energy imparted to the charge carriers is fully utilised. By surrounding area is meant not only the space in the discharge vessel outside the immediate region of origin of the high-frequency oscillations, but also the space outside the vessel walls.

What we claim is:

1. Glow discharge apparatus comprising a metallic chamber, a conductor passing through the wall of the chamber and insulated therefrom, said conductor being adapted to be supplied with a different electrical potential than the said metallic chamber for producing a glow discharge in the chamber, said conductor and wall being hollow to provide cooling spaces therein, a conduit for supplying a cooling medium to one of the spaces, a conduit for withdrawing the heated cooling medium from the other space, and a conduit made of electrically non-conducting material for leading the cooling medium from the first space to the second, whereby the chamher and conductor are connected electrically only through said cooling medium.

2. Glow discharge apparatus comprising, in combina tion, a metallic gas discharge chamber having means for connection to a vacuum pump, electrodes adapted to be connected to a source of electrical potential, and means for suppressing cavity resonance oscillations within the chamber and comprising ribs disposed irregularly in the interior of the chamber.

3. Glow discharge apparatus comprising, in combination, a metallic chamber, a pump connected with the interior of the chamber for evacuating the same to a predetermined degree, electrodes in the interior of the chamber, conductors connecting said electrodes to a source of electric potential for effecting a glow discharge within the chamber, and a low impedance path for high frequency oscillations, and including a condenser connected across the electrodes for suppressing high frequency oscillations generated within the chamber, said chamber being of irregular shape and being provided with irregularly-spaced ribs on the inner wall thereof.

4. Glow discharge apparatus comprising, in combination, a metallic chamber, a pump connected with the interior of the chamber for evacuating the same to a predetermined degree, electrodes in the interior of the chamber, conductors connecting said electrodes to a 9 source of electric potential for eifecting a glow discharge within the chamber, and a low impedance path for high frequency oscillations, and including a condenser connected across the electrodes for suppressing high frequency oscillations generated within the chamber, said electrodes being arranged asymmetrically.

5. Glow discharge apparatus comprising, in combination, a metallic chamber, a pump connected with the interior of the chamber for evacuating the same to a predetermined degree, electrodes in the interior of the chamber, conductors connecting said electrodes to a source of electric potential for effecting a glow discharge within the chamber, a low impedance path for high frequency oscillations, and including a condenser connected across the electrodes for suppressing high frequency oscillations generated within the chamber, at least one of the electrodes, and at least part of the walls of the chamber being hollow to provide cooling spaces, a conduit for supplying one of the spaces with a cooling medium, a conduit for withdrawing the cooling medium from another space, and a conduit of electrically non-conducting material connecting said spaces, whereby the electrode and chamber wall are electrically connected through the high ohmic resistance of the cooling medium.

6. Glow discharge apparatus comprising, in combination, a metallic chamber, a pump connected with the interior of the chamber for evacuating the same to a predetermined degree, electrodes in the interior of the chamber, conductors connecting said electrodes to a source of electric potential for effecting a glow discharge within the chamber, a low impedance path for high frequency oscillations, and including a condenser connected across the electrodes for suppressing high frequency oscillations generated within the chamber, and an electrical condenser connected between at least one of the conductors and the wall of the metallic chamber, said chamber wall being grounded through a tap on the source of potential.

References Cited in the file of this patent UNITED STATES PATENTS 1,905,993 Buff Apr. 25, 1933 2,031,214 Fisher Feb. 18, 1936 2,219,611 Berghaus et al. Oct. 29, 1940 2,454,757 Smith Nov. 23, 1948 2,468,175 Cotton Apr. 26, 1949 

